It's impossible to check each post, that would be a full time job.
But it IS possible to make the forums read-only until you are age-verified. It is already used for the classifieds except in this case folks would still be able to Read the important information AND find out more through the links.
Again, if you can type, then I guess you can read. If you can read, you can find all you need to know on these forums until you are age-verified. If you are from outside Canada, an email to a mod should suffice.
I really want to hear why this would not work, or why a teenager would suddenly say he/she cant cope with this (because, well, they would have read this comment, right? Means they can read more, right?).
The system is already mostly in place (age-verification) to full see a section of the forums, so just make the rest of the forums read-only until verified.
It works elsewhere, quite well too. You should see how calm and idiot-free those forums are. Also how really informative the posts are.